During structural testing, it is often of interest to measure the imposed mechanical strains for either documentary purposes or for control of the test. The most common systems used for strain measurement are extensometers or strain gages, which are devices that are mounted in contact with the test specimen.
In some cases it is desirable to have a non-contact strain measurement system. Such systems are commercially available and typically require registration marks to be placed or projected onto the specimen. The limitation of these systems is that they can only provide strain measurement in a small area, or the strain is averaged over a large area and does not provide information about two-dimensional strain variations throughout the test specimen.
There are many commercially available electro-optical tensile testing strain measurement devices. However, they are not useful in large area measurements.
There is a need for a non-contact strain measurement system that overcomes the limitation of prior systems.